1. Field of the Invention
The present invention relates to a recording and reproduction apparatus, and a recording and reproduction method for recording information to and reproducing information from a recording medium including a plurality of information layers; and such a recording medium.
2. Description of the Related Art
Known optical recording media usable for optical information recording and reproduction include, for example, optical discs and optical cards. Recording information to and reproducing information from an optical recording medium is performed by using a laser light source, such as a semiconductor laser device, and irradiating the optical recording medium with a light beam converged through a lens so as to have a very small diameter.
Technology for further increasing the memory capacity of these optical recording media has been actively developed. Especially, a multi-layered recording medium including a plurality of stacked information layers is capable of doubling, tripling or the like the memory capacity as the number of the information layers is increased by one. In addition, the multi-layered recording medium is easily combined with other high density recording technologies. As a multi-layered recording medium, read only DVD-ROM discs have been practically used. In the future, multi-layered recording media including a plurality of stacked information recording layers formed of a phase change material, a magneto-optic material, a colorant material or the like are expected to be put into practice.
Fundamental technologies for performing recording and reproduction of information to and from an optical recording medium include a focusing servo control for converging a light beam on the optical recording medium using an objective lens, and a tracking servo control for causing a light beam to follow the information tracks. These servo controls are performed based on a signal obtained from light reflected by the optical recording medium and received by a photodetector. When a recording medium including information layers formed of a phase change material or a colorant material is used, signal recording and reproduction is performed using a change in the amount of light reflected by an information layer formed of a thin film. For recording information on such a recording medium, light having a power close to 10 times the power used for reproduction is directed to the recording medium. Accordingly, in accordance with whether the recording track is in a recorded state (where information is recorded therein) or an unrecorded state (where no information is recorded therein) and in accordance with a recording operation is performed or a reproduction operation is performed, the amplitude of a servo control signal varies even though the operation of the objective lens to follow the position of the thin information layer is the same. In order to compensate for such a change in the amplitude of the servo signal, the focusing gain is set to a value in inverse proportion of the value of a summed signal of focusing signals during a focusing operation, and the tracking gain is set to a value in inverse proportion to the value of a summed signal of tracking signals during a tracking operation.
When information is reproduced from a read only multi-layered recording medium including a plurality of information layers, light from the target information layer and also light from adjacent information layers is incident on the reproduction photodetector. The light from the adjacent information layers provides an offset in the servo signal. When reproducing information, the servo signal is compensated for in the direction of the increasing servo gain relative to the case of reproducing information from a recording medium including a single information layer, with the crosstalk from the adjacent information layers being expected.
The multi-layered recording medium has a problem in that when performing recording to and reproduction from the target information layer, the optimum servo conditions change in accordance with the recording state of the adjacent information layers.
For example, when the optical recording medium includes two information layers, there are roughly four states in accordance with the unrecorded/recorded state of each of the two information layers. Here, the information layer closer to the light source is referred to as a “first information layer”, and the information layer farther from the light source is referred to as a “second information layer”. It is now assumed that the first and second information layers are of the form by which the light reflectance is reduced when the state is changed from the unrecorded state to the recorded state. A servo control operation for the second information layer will be described. The focusing gain for the sum of the amounts of light reflected by the second information layer and incident on the focusing photodetector in one state (i.e., the unrecorded state) is in inverse proportion for those in the other state (i.e., the recorded state). The light reflected by the recording medium includes a light component reflected by the second information layer and a light component reflected by the first information layer. The focusing gain is determined based on the sum of the light components. Therefore, the amount of reflected light changes in accordance with whether the first information layer is in the unrecorded or recorded state, even when the target second information layer is in the same state. In consideration of this, the servo gain of the second information layer is adjusted and determined in advance in the state where the first and second information layers are both unrecorded. When information is recorded on the first information layer, and then the area where the servo gain of the second information layer was adjusted in the state where the first and second information layers were both unrecorded is reproduced again, the ratio of the light component reflected by the second information layer is increased since the amount of light reflected by the first information layer is reduced. As a result, the compensation of the servo gain in the sum of the focusing signals is insufficient to compensate for the crosstalk. Thus, the focusing operation is unstable.
The above-described phenomenon also occurs in the tracking servo control operation. Thus, with the conventional multi-layered recording medium, the focusing gain or the tracking gain changes in accordance with the recording state of the information layers adjacent to the target information layer for information recording and reproduction, which destabilizes the servo control operation.